<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    html{
      font-size: 10px;
    }
    .big{
      font-size: 1.4em;
      width: 10em;
      height: 10em;
      background: green;
    }
    .small{
      font-size: 1.2em;
      width: 10em;
      height: 10em;
      /* background: red; */
    }
  </style>
</head>
<body>
  <div class="big">
    我是14px =1.4em
    <div class="small">我是12px=1.em</div>
  </div>
</body>
</html>